Four Senate races in the South that will most likely determine control of Congress appear very close, with Republicans benefiting from more partisan intensity but a Democratic incumbent, once seen as highly vulnerable, holding a surprising edge, according to a New York Times Upshot/Kaiser Family Foundation poll.

The survey underscores a favorable political environment overall for Republicans in Kentucky, North Carolina, Louisiana and Arkansas - states that President Obama lost in 2012 and where his disapproval rating runs as high as 60 percent.

But it also shows how circumstances in each state are keeping them in play for the Democrats a little more than six months before the midterm elections.

Sen. Mark Pryor of Arkansas, a two-term incumbent who has been considered perhaps the most imperiled Democratic senator, holds a 10-point lead over his Republican opponent, Rep. Tom Cotton.

Sen. Kay Hagan, D-N.C., appears more endangered as she seeks a second term. She has the support of 42 percent of voters, and Thom Tillis, the Republican state House speaker and front-runner for his party's nomination, is at 40 percent. And Hagan's approval rating, 44 percent, is the same as her disapproval number.

In Kentucky, Mitch McConnell, the Senate Republican leader, is also effectively tied with his Democratic rival, Alison Lundergan Grimes, a race that may be close because McConnell, first elected to the Senate in 1984, has the approval of only 40 percent of voters, while 52 percent disapprove.

But Grimes must overcome Obama's deep unpopularity in the state, where only 32 percent of voters approve of his performance.

With 42 percent support, Sen. Mary Landrieu, D-La., has an early lead in a race that is not fully formed against a field of three Republicans.

Rep. Bill Cassidy, the Republican front-runner, was the choice of 18 percent, and 20 percent had no opinion. Louisiana has no primary, so all candidates will be on the ballot in November with a runoff in December if needed.

Republicans need to gain six seats to seize the majority and would probably have to defeat at least two of the South's three Democratic incumbents to do so.